Do you mean loose like worn on the inside bore,or loose like in slipping around in the cylinder head? If it's worn in its bore,probably the worst effect would be exhaust smoke,increased oil consumption,and an imprecise valve seal;the valve would eventually burn and leak more,from its rocking in the valve seat and leaking.
If the guide is slipping around in the head,it could break,or stick on the valve stem or in the head,the valve could stick open possibly causing a bent pushrod,or a hole in the piston.
Better to fix the problem as soon as possible,and drive very gently until then.
Speed
